1. Field of the Invention
The invention relates to a magnetic disk apparatus and an optical disk apparatus each having two or more positioning heads and, more particularly, to a magnetic disk apparatus and an optical disk apparatus which are suitable for high density recording.
2. Description of the Related Art
In a disk apparatus, for example, a magnetic disk apparatus, to accomplish high density recording, a sector servo system to position a data head on the basis of a position reference signal recorded in the head of a sector on a data recording surface or a servo system based on such a sector servo system is generally used. Those servo systems based on a sector signal are suitable to position the head to the center of a target track with a high precision. As for a compensator of the above servo systems, since a position signal is discretely obtained, a digital filter is constructed using a microprocessor, and the compensator is determined on the basis of the dynamic characteristic of a control target. The dynamic characteristic of the control target refers to the loop gain from a control signal outputted from the compensator to a detection position signal inputted to the compensator and is given by the product of the gain of a voice coil motor, the gain of an amplifier, the equivalent mass of the head, the position detection gain, and the square of the sampling time.
However, the loop gain of the control target varies in dependence on manufacturing tolerance, an operating condition, an operation environment, and an aging change. Particularly, the gain of the voice coil motor and the position detection gain vary for the following reasons, thereby obstructing the high speed movement of the head and the high accurate positioning operation of the head, so that the performance of the apparatus is deteriorated.
A force constant of the gain of the voice coil motor changes in dependence on the operation track position of the head due to a leakage of the magnetic flux across the voice coil. That is, force constants at the inner and outer peripheries of the disk are small and a force constant at an intermediate position between them is large.
The position detection gain changes due to variations in the peripheral velocity of the disk at which the head is located, the floating amount of the head from the disk surface, and the core width of the head.
That is, since the peripheral velocity on the outer peripheral side of the disk is large, the change in magnetic flux becomes large, a reading voltage of the head becomes large, and the position detection gain increases. When the floating amount of the head is small, the reading voltage becomes also large and the position detection gain also increases. Further, when the core width of the head is larger than a design reference value, the reading voltage becomes large and the position detection gain increases.
Hitherto, means for individually solving the above problems has been proposed. For instance, a method of correcting the change in force constant of the voice coil motor is disclosed in JP-A-63-274395. As an example of the means for totally solving the above problems, a method whereby a loop gain of a mechanism system is automatically estimated during two special speed controls, and compensation elements of the control system are adjusted on the basis of the result of the estimation is disclosed in JP-A-63-23280. Further, a method of obtaining a circular loop gain of an open loop including the voice coil motor and the position detection gain is disclosed in JP-A-2-94187.
Among the above conventional techniques, the technique disclosed in JP-A-63-274395 uses a method whereby correction coefficients corresponding to the position of a head are stored as a table in a memory section in a control apparatus and the correction coefficient is taken out from the table in correspondence to the movement of the head, thereby changing the gain of a compensator. According to such a method, however, since the correction coefficients have already been stored in the table before shipping of the apparatus, a variation of the gain of the voice coil motor of each apparatus cannot be corrected.
Among the above conventional techniques, the technique disclosed in JP-A-63-23280 uses a method whereby an estimation value of loop gain of a mechanism system is calculated by control signals and position signals of one data during the acceleration stage of the speed control and one data during the deceleration stage. Thus, only a rough estimation value can be obtained and a variation of the loop gain between the tracks and a variation of the loop gain of each head cannot be reduced.
As a correcting method of the position detection gain, a method of using an automatic gain control (AGC) amplifier to correct a change in reading voltage due to changes in peripheral velocity of the disk and the head floating amount is known. According to such a method, however, a variation in position detection gain in association with the manufacturing tolerance of the core width of every head cannot be reduced.
Further, among the above conventional techniques, the technique disclosed in JP-A-2-94187 relates to a method of automating a frequency response method whereby a sine wave disturbance of 330 Hz of a zero-cross frequency is added into a servo control loop and the gain in the loop is repetitively adjusted until the ratio of the amplitudes of the sine wave-like signals before and after the addition point is equal to 1. Consequently, the circular loop gain of the open loop can be set to 0 dB by the zero-cross frequency. According to the conventional technique, a point that the above method is applied to all of the heads is shown. According to the above method, however, a fluctuation of the force constant of the voice coil motor due to the operating track position of the head cannot be corrected.
On the other hand, there are the following steps also have been suggested: a step until output signals before and after the addition point are settled to stationary states after a disturbance sine wave was added; a step of measuring an amplitude of each of the sine wave-like signals before and after the addition point of 330 Hz in a state in which the disturbance sine wave has been applied along with subtracting from it the amplitudes of the signals before and after the addition point of 330 Hz in a state in which no disturbance sine wave is applied from the measurement values in order to eliminate a noise component of the signal; and a step of performing a discrete time Fourier transformation on each of the signals before and after the addition point in order to extract only the signal component of 330 Hz of the disturbance sine wave for the above operations. A long time is necessary for each of the above steps. Consequently, there is a problem from viewpoints of time and precision when the automation of the frequency response method is applied to a plurality of heads.
The above matters are important subjects to be solved to realize a high recording density of the disk apparatus, high density installation, high accurate positioning, and high transfer speed.
Although the high recording density can be accomplished by narrowing the track interval and by using a servo system based on a sector signal, in order to realize the narrow track interval, it is necessary to narrow the core width of the head. In association with the narrow core width of the head, there occurs a problem such that the ratio of the manufacturing tolerance to the head core width increases and, consequently, the variation of the position detection gain increases.
Although a high density installation can be accomplished by installing a large number of disk surfaces, in case of the servo system based on the sector signals of the disk surfaces, the positioning heads exist with respect to only the disk surfaces on which the sector signals have been recorded and the position detection gain of each head varies.
For instance, in case of a magnetic disk apparatus of 3.5 inches, which size of disk is typical, when the height of the disk apparatus is equal to 41.3 mm, ten to fourteen positioning heads exist. The manufacturing tolerance of the core width of each positioning head is about 20% when the track interval is set to 2000 TPI (tracks per inch). Further, as a force constant of the voice coil motor, there is a difference of about 10% between the gain of the outer periphery of the disk and the gain of the intermediate position. That is, as a gain fluctuation of a control target in one disk apparatus, the gain fluctuation becomes up to about 30% because of the realization of high recording density and high density installation. Moreover, there is a tendency such that the gain fluctuation of the control target increases due to the realization of high recording density and high density installation of a future disk apparatus.
Although the realization of a high accurate positioning can be accomplished by positioning the head to the center of a target track at a high speed and a high precision, it is necessary to suppress the gain fluctuation (about 30%) of the control target to .+-.4% for this purpose. Particularly, to the head at a high speed and a high precision, it is effective to apply a modern control theory such that the control target is modeled and control means is designed on the basis of such a model. To position the head at a high speed and a high precision, it is necessary to preliminarily accurately know the model of the control target.
Further, the realization of a high transfer speed of data can be accomplished by a method whereby data recorded on a plurality of disk surfaces is simultaneously read and written by all of the heads. For this purpose, after all of the heads were moved in a lump by the voice coil motor, each head needs to be independently positioned to the center of a target track by using a piezoelectric actuator for each head. In such a case, the gain characteristics of piezoelectric element of each head vary and it becomes difficult to position the head at a high speed and with a high precision. It is also necessary to simultaneously estimate the gain characteristics of the piezoelectric element of each head.